Method of removing nitrogen oxides from gaseous mixtures



' p 24, 1962 c. GIAMMARCO 3,031,258

METHOD OF REMOVING NITROGEN OXIDES FROM GASEOUS MIXTURES Filed Feb. 25, 1959 3 Sheets-Sheet 1 Fig. 7

April 24, 1962 G. GIAMMARCO METHOD OF' REMOVING NITROGEN OXIDES FROM GASEOUS MIXTURES 3 Sheets-Sheet 2 Filed Feb. 25, 1959 i@ i W April 24, 1962 G. GIAMMARCO 3,031,258

METHOD OF REMOVING NITROGEN OXIDES FROM GASEOUS MIXTURES Filed Feb. 25, 1959 3 Sheets-Sheet 3 3,031,258 METHOD OF REMOVING NITROGEN OXIDES FROM GASEOUS MIXTURES Giuseppe Giammarco, Porto Karghera, Venice, Italy, assignor to S.p.A. Vetrocoke, Turin, Italy Filed Feb. 25, 1959, Ser. No. 795,434 Claims priority, application Italy Feb. 15, 1955 13 Claims. (Cl. 232) This invention relates to the removal of nitrogen oxides contained by slight traces in gaseous mixtures, more particularly coke oven gas and the like, especially when said mixtures are processed in low temperature fractionating apparatus.

It is known from industrial practice that in fractionating apparatus, in which a gas such as coke oven gas is decomposed to its components by a progressive liquefaction at low temperature and at superatmospheric pressure, the presence of nitrogen oxides in the gaseous mixture, even by small traces as is normally the case, that is fractions of parts per million to a few parts per million, is objectionable in that such nitrogen oxides react with unsaturated hydrocarbons of the diene class which are present in the gaseous mixture thereby forming gums as the reaction product. Said gums, which may even be of an explosive nature, deposit within the apparatus. Such nired States Patent deposits are known to be objectionable in practice under various aspects, more particularly the danger of explosions. For the above reasons the operational life of fractionating apparatus even of the most up-to-date type does not last for reasons of technique and safety longer than 1 to 3 months. Consequently, the apparatus should be periodically stopped in order to remove the gum deposits by washing by means of suitable solvents, which necessitates in the end the use of a reserve apparatus during the stop period.

Even in other technical fields the presence of nitrogen oxides in gaseous mixtures together with unsaturated hydrocarbons has led to various drawbacks, such as clogging of measuring apparatus and pipes etc.

The object of this invention is to effect fine removal of nitrogen oxides from the gaseous mixtures containing them.

A further object is to prevent said nitrogen oxides from forming gums, more particularly explosive gums.

A further object is to prevent in apparatus processing mixtures containing nitrogen oxides and unsaturated hydrocarbons clogging due to formation of gums, thereby avoiding the danger of explosions.

A further object is to avoid periodical stoppage of fractionating apparatus, thereby dispensing with reserve apparatus.

The reaction of nitrogen oxides and unsaturated hydrocarbons, which is the cause of the above described disturbances was utilized by a number of attempts at removing nitrogen oxides by effecting said reaction in a preliminary apparatus arranged, for instance, ahead of the access of gas to pipings from which it issues for use or ahead of the fractionating apparatus etc., however, such attempts were industrially unsuccessful.

In this connection reference is made to the method of removing nitrogen oxides by treatment of a gaseous mixture containing them together with unsaturated hydrocarbons, more especially when processing coke oven gas, by means of electric discharges, which promotes the reaction in a gaseous phase of nitrogen oxides and unsatu rated hydrocarbons which results in the formation of gums and subsequent deposition thereof.

A further known method consists in treating nitrogen oxides and unsaturated hydrocarbons by means of activated carbon or other adsorbent materials, which catalyze said reaction, the product of which is thereafter decomposed to nitrogen and C0 The above methods did not lead in industrial to complete or satisfactory results.

The improved method is based on novel characteristics and aspects which were not utilized heretofore, of the reaction of the nitrogen oxides and unsaturated hydrocarbons, through which the problem has been fully solved, so that the improved method affords an unlimited duration of the fractionating apparatus to the point that a large number of investigations carried out after 30 months operation and even longer detected no gum formation therein.

It is known from long experience in the coke oven gas industry that mere co-existence in said gas of nitrogen oxides and unsaturated hydrocarbons results in a very coarse and unsatisfactory removal of nitrogen oxides so that in said industry, even after repeated washing of the gas through various apparatus for removing tar, naphpractice thalene, benzene and the like, nitrogen oxides still per sist by an appreciable amount while, more particularly in respect of the fractionation of gas in low temperature apparatus, nitrogen oxides should be removed as thoroughly as possible.

I have now found thatreaction of nitrogen oxides and unsaturated hydrocarbons of the diene class in the gaseous phase should be avoided because said reaction is not only incomplete, but forms in a gaseous phase compounds in which nitrogen oxides are left occluded and which remain in suspension in the gaseous stream and subsequently deposit in pipes, meters or in low temperature fractionating apparatus with a consequent danger. Moreover, I have found that nitrogen oxides contained in the said reaction products are occluded in such manner that they cannot be removed by subsequent treatment, such as washing by means of oxidizing and other liquids, through which they travel without being absorbed.

I have further found that this drawback may be removed by heating the gaseous stream in which said reaction products are suspended at least above 60 C., preferably within the range of -450 0., said heating decomposing the reaction product, setting free again nitrogen oxides, said nitrogen oxides being capable of subsequent removal by absorption by means of suitable agent in accordance with this invention. I have further found that the reaction of nitrogen oxides and unsaturated hydrocarbons of the diene class takes place much more satisfactorily in a liquid phase, by causing the nitrogen oxides to be absorbed by a liquid consisting of unsaturated hydrocarbons in a pure state, or a liquid solvent containing high concentration of said hydrocarbons, which react in a liquid phase with the nitrogen oxides, so that the reaction products remain in a liquid state and are no longer carried away by the gaseous stream.

I have further found that unsaturated hydrocarbons most effective in respect with removal of nitrogen oxides are especially those of the diene class, the boiling point of which ranges between 35 and 45 C., sm'd range including the cyclopentadiene which is the most abundant and active among them.

I have ascertained that the high volatility of unsaturated hydrocarbons mentioned above requires the use of superatmospheric pressures and sufficiently low temperatures for said hydrocarbons to remain in a liquid state or in a strongly concentrated solution in the liquid solvent; moreover, I have ascertained that the volatility of unsaturated hydrocarbons mentioned above necessitates a special operational cycle in which a stage or compartment for removing nitrogen oxides by contact with unsaturated hydrocarbons is followed by a further stage referred to as recovery zone in which the unsaturated hydrocarbons which could be volatilized and carried away by the gas stream are recovered from the latter and returned to the first named stage of removal of the nitrogen oxides.

The above broad statements shall be detailed hereafter by reporting the experiments and investigations carried out for this purpose.

As mentioned above, I have found in industrial practice that a gas mixture in which nitrogen oxides and unsaturated hydrocarbons co-exist exhibits in chemical analysis a variable behaviour depending upon whether it is analyzed as such or is previously heated to a temperature exceeding 60 C., preferably a temperature ranging between 90-150" C. during a period of time which is shorter as the heating temperature is higher; more particularly I have found that analysis carried out after heating constantly detect the presence of a considerably larger quantity of nitrogen oxides. For instance, a purified coke oven gas drawn at the inlet of a fir-actionating apparatus, analyzed by the known method consisting in oxidizing nitrogen oxides to N by means of a permanganate solution, hence in determining N0 by a colorimetric procedure, exhibited a nitrogen oxide content of 0.5-0.8 part per million, while, on heating the gas prior to anal ysis, the nitrogen oxide contents was found to be 1.5-3 parts per million. This is all the more evident on considering a gaseous mixture consisting of the so called ethylenic fraction forming in a fractionating apparatus from coke oven gas, that is, the fraction recovered in that section of the apparatus in which ethylene is condensed and in which nitrogen oxides are preferentially concentrated. This fraction exhibits on analysis by the abovementioned method about 4 parts per million nitrogen oxides while, if heated to 125 C. prior to analysis, it exhibited a nitrogen oxide content of 18 parts per million. The circumstance that the gaseous mixture contains nitrogen oxides which are not detected by highly oxidizing solutions such as a permanganate solution shows that nitrogen oxides themselves are present as a combination which is uncapable of reacting, that is they are contained as an unstable compound with unsaturated polymerizable hydrocarbons contained in the gas, said compound, decomposable on heating, being kept in suspension in the gaseous stream in the form of vapour or mist.

This circumstance is of the utmost importance, for it explains the reason why the method of removing nitrogen oxides by reacting them with unsaturated hydrocarbons in a gaseous state never led to satisfactory results.

The circumstance that removal of nitrogen oxides is complete when employing as an absorption agent unsaturated hydrocarbons of the diene class in a liquid condition was ascertained over a larger number of tests carried out before actually practicing the improved method on an industrial scale. On a typical test a gas mixture containing nitrogen oxides by the small quantities as usually found in actual practice in gaseous mixtures, in the specific case 2-3 parts per million, is scrubbed by means of a liquid consisting of kerosene having dissolved therein cyclopentadiene, operating at a temperature of -20 C. and a pressure of 12 atm. After the treatment the gaseous mixture was found to be practically free from nitrogen oxides from the analysis carried out after a preliminary heating of the mixture. On a similar test carried out under similar conditions, in which the kerosene did not contain any cyclopentadiene, the nitrogen oxides remained practically unaltered in the gaseous mixture, which discloses that removal of nitrogen oxides does not take place through solubility thereof in the solvent at low temperature and superatmospheric pressure, but was due on the first test to a reaction with unsaturated hydrocarbons contained in the solvent.

Under conditions similar to those adopted on the above described test the vapour pressure disclosed by the cyclopentadiene present in a 5% dilution in the solution results in that the gaseous mixture contacting the solution will remove therefrom and carry away about 500-600 parts per million of cyclopentadiene, which is a quantity considerably disproportionate as compared with the small quantities of nitrogen oxides to be removed. Consequently, from a practical and industrial standpoint a suitable operational cycle should be provided for recovering said unsaturated hydrocarbons and subsequently re-utilizing them.

In accordance with the above removal of nitrogen oxides by the improved method takes place, instead of in the gaseous phase, by reaction in a liquid phase of said nitrogen oxides with unsaturated hydrocarbons of the diene class, the boiling point of which ranges between 35 and 45 C., under suitable superatmospheric pressure and low temperature conditions for said unsaturated hydrocarbons to be present in a liquid state or in a dissolved condition in a sufiiciently high concentration, the unsaturated hydrocarbons removed volatilized and carried along by the gaseous stream being recovered and recycled.

This improvement therefore broadly consists in causing the gaseous mixture to be purified to travel through two consecutive zones, namely a first zone at which nitrogen oxides are removed, and a successive zone at which unsaturated hydrocarbons are recovered, and providing an operational system for re-cycling the recovered unsaturated hydrocarbons, as well as the necessary measure for removing any undesirable impurities as may accumulate in the system.

Referring to FIGURE 1, which shows the general operational diagram of the method, the gaseous mixture is introduced into the zone 1 for removal of nitrogen oxides, at which it is contacted by unsaturated hydrocarbons maintained in a liquid condition. Sufliciently low temperatures and sufficiently high superatmospheric pressures are employed at this zone for said hydrocarbons to be present in a liquid condition, the nitrogen oxides reacting in a liquid phase with the unsaturated hydrocarbons and being removed avoiding any formation of volatile reaction compounds. The gaseous mixture which has been purified from nitrogen oxides and contains unsaturated hydrocarbon vapours is conveyed to the successive zone 2 for recovery of unsaturated hydrocarbons, at which it is scrubbed by means of a liquid solvent that absorbs unsaturated hydrocarbons therein contained. Said liquid solvent is drawn from the zone 2 and conveyed to a contiguous regeneration zone 3, at which unsaturated hydrocarbons are expelled from the liquid solvent by heating or otherwise and returned to the zone 1 for removal of the nitrogen oxides, the purified liquid solvent returning to the recovery zone 2.

The necessary quantity of unsaturated hydrocarbons is supplied to zone 1 as required for making up for any losses in the cycle, the hydrocarbon charge being renewed or purified as it becomes enriched with impurities and reaction products with nitrogen oxides.

The liquid solvent employed at zone 2 is a generic liquid capable of dissolving unsaturated hydrocarbons, and shall be defined in greater detail hereafter.

V The operational cycle as shown by FIGURE 1, which should be understood as the basic scheme of the method, is generally applicable, while the expert shall in the individual instances from his knowledge of the vapour tension of unsaturated hydrocarbons employed for removing nitrogen oxides determine the necessary pressure and temperature conditions at the first zone for keeping said hydrocarbons in a liquid condition.

Referring to the above referred experiences it was found that the improved process according to the cycle shown in FIGURE 1 is satisfactorily carried out even when unsaturated hydrocarbons are employed, instead of in a pure liquid condition, dissolved to a sufficiently high concentration in an organic solvent. The lowest concentration of unsaturated hydrocarbons required was found equal at least 0.5%, while higher concentrations are preferable, especially where a very thorough purification from nitrogen oxides is required; at any rate, solutions of higher unsaturated hydrocarbon concentration considerably reduce the volume of the nitrogen oxides removal-zone. The use of a liquid solvent is convenient for said solvent by diluting unsaturated hydrocarbons lowers the vapour pressure thereof making available hydrocarbons in a liquid condition at more convenient pressures and temperatures. The liquid solvent may be selected among any organic liquids capable of mixing with and dissolving unsaturated hydrocarbons, provided the said liquids are of a low freezing point, preferably of low volatility. I may quote by Way of example kerosene, petroleum, gasolines, solvent naphtha, chlorinated hydrocarbons, mixtures thereof, etc. The improved method preferably employs kerosene on account of its low cost and low volatility.

When employing an unsaturated hydrocarbon solution in a solvent it was found that at the nitrogen oxide removal zone a pressure should be employed which exceeds 4 atm. at least and a temperature below 20 C. at least, preferably a pressure ranging between and atm. and a temperature ranging between and C. a

The use of a liquid solvent containing unsaturated hydrocarbons in a sufiiciently high concentration is preferable where the gaseous mixture to be purified contains in addition to nitrogen oxides unsaturated hydrocarbons as is the case with coke oven gas and in a large number of other important industrial processes. The liquid solvent is then utilised for absorbing unsaturated hydrocarbons contained in the gaseous mixture, which are subsequently used for removing nitrogen oxides, the operational scheme disclosed by FIGURE 1 remaining substantially unaltered, but for adaptations to practical requirements, Where modifications are advisable, such as shown by way of a non limiting example in FIGURES 2 and 3.

Since the gaseous mixture contains both nitrogen oxides and unsaturated hydrocarbons, in accordance with the above the gaseous mixture is conveniently heated prior to removal of nitrogen oxides, to a temperature of at least C., preferably to a temperature ranging between 90 and 150 C., in order to decompose the compounds in which nitrogen oxides are occluded, whereby the latter are more easily removable. A heating zone is therefore arranged ahead of the nitrogen oxide removal zone 1, which warrants that nitrogen oxides are suitable for absorption on the next step.

A further adaptation of the basic circuit according to FIGURE 1, becomes practically necessary through the circumstances that the liquid solvent employed at the nitrogen oxide removal zone often becomes loaded with various undesirable impurities, such as benzene, toluene and the like, contained in the gas to be purified, which should be separated and discharged to the outside. This can be taken care of for instance by a rectifying apparatus. However, during removal of said undesirable. impurities, useful unsaturated hydrocarbons for removing nitrogen oxides should not be expelled.

The use of this method of removing nitrogen oxides from a gaseous mixture containing few unsaturated hydrocarbons of the diene class is shown by FIGURE 2. This is the most frequent case, considering that coke oven gas mostly contains on inlet to a low-temperature fractionating apparatus small quantities only ofcyclopentadiene (20-30 parts per million), on account of the fact that it has lost on its preliminary purifications most of its content of such hydrocarbon. Under these conditions, a 0.5% concentration of cyclopentadiene in a solvent such as ,kerosene, which concentration is the lowest one required for removing nitrogen oxides, can be attained at the equilibrium only by employing 12 atm. pressure and a temperature of 4=Q C., while it is attained in industrial practice only through a recovery of said cyclopentadiene and recycling thereof.

Referring to FIGURE 2, a coke oven gas in a compressed condition containing nitrogen oxides and unsaturated hydrocarbons is heated in a suitable apparatus 4 in order to set free nitrogen oxides to render them suitable for easy removal and is conveyed upon cooling to a tower 6 in which it is scrubbed by means of a cool liquid solvent containing a sufi'icient proportion of unsaturated hydrocarbons of the diene class, distinguishing through a boiling point ranging between 35 and 45, such a solution being circulated in a closed cycle by a pump 7. The solution absorbs nitrogen oxides which react within said solution with the abovementioned hydrocarbons in a liquid phase and are thereby removed. The gas, which is practically free from nitrogen oxides and contains a proportion of unsaturated hydrocarbons corresponding to the vapour pressure of the latter at the temperature and pressure employed, then flows to a tower 8 in which it is scrubbed by means of a fresh solvent in order to recover unsaturated hydrocarbons. Finally, the gas free from both nitrogen oxides and unsaturated hydrocarbons issues at the top of the tower 8 and is conveyed to its place of use, such as to fractionating apparatus. Before utilizing the gas, the latter is conveniently caused to travel over activated carbon in order to purify it from the solvent vapours therein contained on issue from the scrubbing tower 8. The solvent, which has become loaded in tower 8 with unsaturated hydrocarbons recovered from the gas, is drawn at the bottom of the tower through valve 9 and, upon yielding its cold in the heat exchanger 10, is conveyed to the distilling column 11 heated at 12 by means of indirect steam, said column being equipped at the top with a water-cooled dephlegmator. The liquid solvent deprived by distillation of the unsaturated hydrocarbons issues from the column 11, is cooled in the water cooler 14, further cooled in the heat exchanger 10 and pumped by a pump 15 upon cooling to low temperature at 16 again to the top of the tower 8. The unsaturated hydrocarbon vapours issuing at the top of the distilling column 11 are conveyed to the condenser 17, next to the condenser 18 in which unsaturated hydrocarbons are condensed and conveyed to the liquid circulated in a closed cycle through the tower 6 in order to maintain the desired high concentration in the latter.

FIGURE 2 provides a further measure suggested by experience made in actual practice in order to remove from the liquid circulated in a closed circuit in the tower 6 the various impurities, such as benzene, toluene and the like contained in the gaseous mixture with which the liquid becomes loaded. The regeneration apparatus 11 itself can be utilized for removing said impurities by continuously or discontinuously supplying thereto part of the liquid solvent circulating in tower 6 and substituting for the latter a corresponding quantity of the liquid in tower 8 drawn through valve 19. The part liquid circulating through 6, which should be purified, is drawn at 20 and conveyed together with the part solvent from 8 through 10 to the distilling column 11. The temperature at the top of the column is adjusted in such manner as to free the solvent, in addiiton to unsaturated hydrocarbons boiling' between 35 and 45 C., of benzene, toluene or any further compound boiling below 120 C. The vapours issuing from column 11 flowing to the condenser 17 liquefy undesirable impurities such as benzene, etc. which are discharged, while remaining vapours containing unsaturated hydrocarbons are condensed at 18, as mentioned above and recovered.

The abovedescribed cycle is employed for protecting large fractionating apparatus processing coke oven gas which after undergoing successive purificationbefore reaching the fractionating apparatus, contains about 20 parts per million cyclopentadiene and about 0.5 part per million nitrogen oxides. At the nitrogen oxide removal zone a pressure of 12 atm. and a temperature of 40 C. are employed, kerosene being used as a solvent.

A further special use of the improved method is shown by FIGURE 3 and applies when the gas to be treated for removing nitrogen oxides therefrom is appreciably rich in unsaturated hydrocarbons, which need therefore not be recovered by providing a tower 8 in FIGURE 2, the basic cycle being then simplified by dispensing with the tower 8. The method consequently essentially requires a nitrogen oxide removal zone only, at which the liquid solvent, when it should be regenerated for expelling any undesirable impurities mentioned above, is processed to remove the latter only, the unsaturated hydrocarbons being returned to the cycle. FIGURE 3 shows this particular procedure.

Similarly to the previously described embodiment, the compressed gas is pre-heated at 21, cooled at 22 and conveyed to a tower 23 in which it is scrubbed by means of a liquid solvent containing unsaturated hydrocarbons in a r ifficient concentration. The gas freed from nitrogen oxides issues at the top of the tower 23 and is conveyed to its place of use, possibly after an adsorption step by activated carbon of other adsorbing materials. The liquid solvent containing unsaturated hydrocarbons is circulated in a closed cycle by a pump 24. In order to remove undesirable impurities, such as benzene, toluene, etc., with which the liquid becomes loaded in contact with the gas, part thereof is drawn in this case also from the bottom of the tower 23 and conveyed through 25 upon a heat exchange at 26 to the distilling column 27 heated at 28 by means of steam and equipped with a dephlegmator 29. The temperature at the top of the column 27 is adjusted in such manner as to purify the solvent from impurities or any compound boiling below 80-120 C. The purified liquid solvent issuing at the bottom of the column 27, is cooled at 30 and 26 and collects in the container 31 whence it is conveyed by means of the pump 32 after further cooling at 33 to the top of the tower 23. The vapours issuing at the top of the column 27 containing both impurities and unsaturated hydrocarbons are conveyed to the condenser 34 in which impurities such as benzene, etc. are condensed; the remaining vapours, mostly consisting of the unsaturated hydrocarbon fraction the boiling point of which ranges between 35 and 45 C., which should be recovered and returned to the solvent, are condensed in a successive condenser 35 and conveyed to the collector 31.

This procedure was likewise carried out in large fractionating apparatus with highly satisfactory results; the apparatus were found to be of a practically indefinite operational life, on washing with alkaline lyes, their internal circuits were found to be free from any gum deposit due to the presence of nitrogen oxides on a larger number of inspections carried out from time to time during 30 months uninterrupted operation.

For instance, in a specific case 24,000 Nobmjh. gas compressed at 12 atm. containing 0.5 part per million nitrogen oxides is scrubbed by means of illuminating petroleum containing unsaturated hydrocarbons of the diene class at a temperature of -40 C. The scrubbing liquid circulating in a short-circuit is about 20 cbm./h., the part liquid conveyed to distillation being 2-4 cbm./ h.

When the reaction products of the nitrogen oxides and unsaturated hydrocarbons forming within the liquid and withheld by the latter during the practice of the improved method become accumulated, part of the liquid should be removed and replaced by fresh liquid. Moreover, in order to prevent reaction products from polymerizing within the liquid, the latter is conveniently admixed with anti-polymerizing agents, such as hydroquinone and the like. This is a continuation-in-part of my application Serial No. 565,717, filed February 15, 1956, and now abandoned.

What I claim is:

1. Method of removing nitrogen oxides from a gaseous mixture comprising the steps of providing a washing zone for the mixture, washing the mixture in said zone with an unsaturated hydrocarbon of the diene class having a boiling point within the range of 35 C. and 45 C. while maintaining in the zone a pressure ranging between and atmospheres and a temperature ranging between -30 C. and -50 C. effective to maintain said unsaturated hydrocarbon in the liquid phase for reaction with the nitrogen oxides, then washing the mixture with a liquid solvent for said hydrocarbon in a subsequent recovery zone thereby to absorb from the mixture vapors of the hydrocarbon, drawing from the recovery zone the liquid solvent loaded with the absorbed hydrocarbon to a regeneration zone for said solvent, regenerating the solvent in the regeneration zone by expelling the absorbed hydrocarbon from the solvent, recirculating the expelled hydrocarbon to said washing zone and recirculating the regenerated solvent to said recovery zone.

2. Method of removing nitrogen oxides from a gaseous mixture containing also an unsaturated hydrocarbon of the diene class having a boiling point within the range of 35 C. and C., comprising the steps of heating the mixture to a temperature of 60 C. to 150 C. to thereby decompose any reaction product of the nitrogen oxides with the hydrocarbon in the mixture, then conveying the mixture through a washing zone while circulating through the zone a liquid solvent containing said hydrocarbon dissolved therein at a concentration of at least 0.5% and while maintaining in the zone a pressure ranging between l0 and 20 atmospheres and a temperature ranging between 30 C. and C. effective to maintain the hydrocarbon in the liquid solvent at said concentration to react with the nitrogen oxides in the mixture flowing through the zone, drawing the mixture from the washing zone through a recovery zone while scrubbing the mixture in the recovery zone with fresh solvent for said hydrocarbon thereby to absorb from the mixture vapors of the hydrocarbon, withdrawing from the recovery zone hydrocarbon-loaded liquid solvent and regenerating the latter by expelling therefrom said hydrocarbon, recirculating the regenerated solvent to the recovery zone, and recirculating the expelled hydrocarbon to the solvent circulating through the washing zone.

3. Method of removing nitrogen oxides from a gaseous mixture containing also an unsaturated hydrocarbon of the diene class having a boiling point within the range of 35 C. and 45 C. and containing other hydrocarbon impurities, comprising the steps of heating the mixture to a temperature of C. to C. to thereby decompose any reaction product of the nitrogen oxides with the unsaturated hydrocarbon in the mixture, then conveying the mixture through a washing zone While circulating through the zone a liquid solvent containing said unsaturated hydrocarbon dissolved therein at a concentration of at least 0.5% and while maintaining in the zone a pressure ranging between 10 and 20 atmospheres and a temperature ranging between -30 C. and 50 C. effective to maintain the unsaturated hydrocarbon in the liquid solvent at said concentration to thereby react with the nitrogen oxides in the mixture flowing through the zone, drawing the mixture from the washing zone through a recovery zone while scrubbing the mixture in the recovery zone with fresh solvent for said hydrocarbon thereby to absorb from the mixture vapors of the unsaturated hydrocarbon, withdrawing from the recovery zone a proportion of unsaturated hydrocarbon-loaded liquid solvent and conveymg said proportion to the washing zone for washing the gaseous mixture in the latter, withdrawing from the recovery zone a further proportion of unsaturated hydrocarbon-loaded liquid solvent, withdrawing from the washmg zone a proportion of fouled liquid solvent, jointly regenerating the last-named proportion and said further proportion of unsaturated hydrocarbon-loaded liquid solvent by removing therefrom said impurities and said unsaturated hydrocarbon, recovering the removed unsaturated hydrocarbon and recirculating it to the liquid solvent circulating through the washing zone, and recovering the regeneration solvent and recirculating it to the recovery zone for scrubbing the mixture in the latter.

4. Method of removing nitrogen oxides from a gaseous mixture containing also an unsaturated hydrocarbon of the diene class having a boiling point within the range of between 35 C. and 45 C., and containing other hydrocarbon impurities, comprising the steps of heating the mixture to a temperature of 60 C. to 150 C. to thereby decompose any reaction product of the nitrogen oxides with the unsaturated hydrocarbon in the mixture, then conveying the mixture through a washing zone while circulating through the zone a liquid solvent for said unsaturated hydrocarbon and said impurities, said solvent containing said unsaturated hydrocarbon dissolved therein, and while maintaining in the zone a pressure ranging between 10 and 20 atmospheres and a temperature ranging between 30" C. and 50 C. effective to absorb and retain unsaturated hydrocarbon in the liquid solvent at a concentration of at least 0.5% to thereby react with the nitrogen oxides carried by the mixture flowing through the zone, drawing from the washing Zone a proportion of fouled liquid solvent, removing from said proportion the impurities and unsaturated hydrocarbon and recirculating the solvent and unsaturated hydrocarbon to the washing zone.

5. Method of removing nitrogen oxides from a gaseous mixture comprising the steps of providing a washing zone for the mixture, Washing the mixture in said zone with an unsaturated hydrocarbon of the diene class having a boiling point within the range of 35 C. and 45 C. while maintaining in the zone a pressure ranging between 4 and 20 atmospheres and a temperature ranging between -20 C. and 50 C. effective to maintain said unsaturated hydrocarbon in the liquid phase for reaction with the nitrogen oxides, then washing the mixture with a liquid solvent for said hydrocarbon in a subsequent recovery zone thereby to absorb from the mixture vapors of the hydrocarbon, drawing from the recovery zone the liquid solvent loaded with the absorbed hydrocarbon to a regeneration zone for said solvent, regenerating the solvent in the regeneration zone by expelling the absorbed hydrocarbon from the solvent, recirculating the expelled hydrocarbon to said waslnng zone and recirculating the regenerated solvent to said recovery zone.

6. Method of removing nitrogen oxides from a gaseous mixture containing also an unsaturated hydrocarbon of the diene class having a boiling point within the range of 35 C. and 45 C., comprising the steps of heating the mixture to a temperature of 60 C. to 150 C. to thereby decompose any reaction product of the nitrogen oxides with the hydrocarbon in the mixture, then conveying the mixture through a washing zone while circulating through the zone a liquid solvent containing said hydrocarbon dissolved therein at a concentration of at least 0.5% and while maintaining in the zone a pressure ranging between 4 and 20 atmospheres and a temperature ranging between 20 C. and '-50 C. effective to maintain the hydrocarbon in the liquid solvent at said concentration to react with the nitrogen oxides in the mixture flowing through the zone, drawing the mixture from the washing zone through a recovery zone while scrubbing the mixture in the recovery zone with fresh solvent for said hydrocarbon thereby to absorb from the mixture vapors of the hydrocarbon, withdrawing from the recovery zone hydrocarbon-loaded liquid solvent and regenerating the latter by expelling therefrom said hydrocarbon, recirculating the regenerated solvent to the recovery zone, and recirculating the expelled hydrocarbon to the solvent circulating through the washing zone.

7. Method of removing nitrogen oxides from a gaseous mixture containing also an unsaturated hydrocarbon of the diene class having a boiling point within the range V of 35 C. and 45 C. and containing other hydrocarbon impurities, comprising the steps of heating the mixture to a temperature of 60 C. to 150 C. to thereby decompose any reaction product of the nitrogen oxides with the unsaturated hydrocarbon in the mixture, then conveying the mixture through a washing zone while circulating through the zone a liquid solvent containing said unsaturated hydrocarbon dissolved therein at a concentration of at least 0.5% and while maintaining in the zone a pressure ranging between 4 and 20 atmospheres and a temperature ranging between 20 C. and 50 C. effective to maintain the unsaturated hydrocarbon in the liquid solvent at said concentration to thereby react with the nitrogen oxides in the mixture flowing through the zone, drawing the mixture from the washing zone through a recovery zone while scrubbing the mixture in the recovery zone with fresh. solvent for said hydrocarbon thereby to absorb from the mixture vapors of the unsaturated hydrocarbon, withdrawing from the recovery zone a proportion of unsaturated hydrocarbon-loaded liquid solvent and conveying said proportion to the washing zone for washing the gaseous mixture in the latter, withdrawing from the recovery zone a further proportion of unsaturated hydrocarbon-loaded liquid solvent, withdrawing from the washing zone a proportion of fouled liquid solvent, jointly regenerating the last-named proportion and said further proportion of unsaturated hydrocarbonloaded liquid solvent by removing therefrom said im purities and said unsaturated hydrocarbon, recovering the removed unsaturated hydrocarbon and recirculating it to the liquid solvent circulating thnough the washing zone, and recovering the regeneration solvent and recirculating it to the recovery zone for scrubbing the mixture in the latter.

8. Method of removing nitrogen oxides from a gaseous mixture containing also an unsaturated hydrocarbon of the diene class having a boiling point within the range of between 35 C. and 45 C., and containing other hydrocarbon impurities, comprising the steps of heating the mixture to a temperature of 60 C. to C. to thereby decompose any reaction product of the nitrogen oxide with the unsaturated hydrocarbon in the mixture, then conveying the mixture through a washing zone while circulating through the zone a liquid solvent for said unsaturated hydrocarbon and said impurities, said solvent containing said unsaturated hydrocarbon dissolved therein, and while maintaining in the zone a pressure ranging between 4 and 20 atmospheres and a temperature ranging between 20 C. and 50 C. eiiective to absorb and retain unsaturated hydrocarbon in the liquid solvent at a concentration of at least 0.5% to thereby react with the nitrogen oxides carried by the mixture flowing through the zone, drawing from the washing zone a proportion of fouled liquid solvent, removing from said proportion the impurities and unsaturated hydrocarbon and recirculating the solvent and unsaturated hydrocarbon to the washing zone.

9. In the method as defined in claim 5, Washing the mixture containing nitrogen oxides with said hydrocarbon dissolved in said solvent at a concentration of at least 0.5%.

10. The method as defined in claim 5, wherein said hydrocarbon is cyclopentadiene.

11. The method as defined in claim 6, wherein said hydrocarbon is cyclopentadiene.

12. The method as defined in claim 7, wherein said unsaturated hydrocarbon is cyclopentadiene.

13. The method as defined in claim 8, wherein said unsaturated hydrocarbon is cyclopentadiene.

References Cited in the file of this patent FOREIGN PATENTS 307,886 Great Britain July '15, 1930 336,456 Great Britain Oct. 16, 1930 483,706 Great Britain Apr. 21, 1938 

5. METHOD OF REMOVING NITROGEN OXIDES FROM A GASEOUS MIXTURE COMPRISING THE STEPS OF PROVIDING A WASHING ZONE FOR THE MIXTURE, WASHING THE MIXTURE IN SAID ZONE WITH AN UNSATURATED HYDROCARBON OF THE DIENE CLASS HAVING A BOILING POINT WITHIN THE RANGE OF 35*C. AND 45*C. WHILE MAINTAINING IN THE ZONE A PRESSURE RANGING BETWEEN 4 AND 20 ATMOSPHERES AND A TEMPERATURE RANGING BETWEEN -20*C. AND -50*C. EFFECTIVE TO MAINTAIN SAID UNSATURATED HYDROCARABON IN THE LIQUID PHASE FOR REACTION WITH THE NITROGEN OXIDES, THE WASHING THE MIXTURE WITH A LIQUID SOLVENT FOR SAID HYDROCARABON IN A SUBSEQUENT RECOVERY ZONE THEREBY TO ABSORB FROM THE MIXTURE VAPORS OF THE HYDROCARBON, DRAWING FROM THE RECOVERY ZONE THE LIQUID SOLVENT LOADED WITH THE ABSORBED HYDROCARBON TO A REGENERATION ZONE FOR SAID SOLVENT, REGENERATING THE SOLVENT IN THE REGENERATION ZONE BY EXPELLING THE ABSORBED HYDROCARBON FROM THE SOLVENT, RECIRCULATING THE EXPELLED 